Couplers which are installed on electrical machinery, such as generators, motors, or transformers, for diagnosing the condition of machinery insulation must accurately extract the required electrical signals without compromising machinery performance or degrading the signals of interest. For coupling partial discharge analysis (PDA) testing equipment with electrical machinery, a convenient technique is coupling off the ground-strap to a surge capacitor used for protecting the equipment against extraneous surges from sources such as lightning. Surge capacitors are generally oil-filled capacitors which have the potential of creating a fire or explosion. Most oil-filled surge capacitors are not sufficiently corona-"free", and therefore the capacitor itself can create discharges which must be considered when analyzing the data. The oil-filled surge capacitors that are designed to be corona-free are expensive. Many installations do not have surge capacitors and suitable capacitors must be installed. As these capacitors are continuously energized, they must be reliable so that they do not affect the integrity of the system. Surge capacitors generally attenuate and do not sufficiently pass high frequency signals.
In aforementioned Krahn et al., U.S. application Ser. No. 08/970,464, a partial discharge (PD) coupler for coupling electrical machinery to partial discharge analysis testing equipment includes at least one capacitor capable of withstanding high voltages; potting material surrounding the at least one capacitor for providing flashover protection; and a fuse for protecting the electrical machinery, the fuse physically and electrically connected in series to the at least one capacitor, the at least one capacitor and the fuse capable of passing high frequency partial discharges from the electrical machinery to the partial discharge analysis testing equipment with minimal attenuation. The potting material may include an elastomeric or thermoset material and in one embodiment comprises silicone. The PD coupler can have potting material surrounding both the capacitors and the fuse. The PD coupler can include a housing surrounding the capacitors, the potting material, and the fuse. This embodiment provides an efficient method and a corona-free apparatus for inexpensively providing safe and effective connection of partial discharge equipment to electric devices.